1. Field of the Invention
The present invention relates to a bone conduction earphone, and more specifically relates to a bone conduction earphone which can quickly and accurately reflect a small change of electric current in a bone conducted vibration.
2. Description of the Background Art
Normally, a vibration of sound conducted through air is transmitted to an eardrum inside an ear. There are three bones (ossicles) inside the eardrum, and a vibration of the eardrum is transmitted to a screw-like cochlea via the bones. The cochlea contains lymph, and a vibration of the lymph is transmitted to the auditory nerve as electric signals, whereby a brain recognizes the sound. On the other hand, in a bone conduction, the process of the vibration travelling through the eardrum and the ossicles in the mechanism described above is omitted. That is, the vibration of sound is directly sent to the cochlea via a skull, whereby even a hearing-impaired person and the like having an abnormality in the eardrum or ossicles can reliably hear the sound by the bone conduction as long as the cochlea and the auditory nerve are normal. An earphone applying this principle is the bone conduction earphone.
Various types and forms of bone conduction earphones have been conventionally developed. Most of them are configured of a frame, a voice coil, a magnet, a diaphragm, and the like, and have a structure in which the magnet is arranged in either an inner side region or an outer side region of the voice coil (for example, see Patent Document 1). However, magnetic forces generated by the conventional bone conduction earphones are limited, whereby a quick and accurate response cannot be expected unless an accordingly large amount of electric current is supplied to the voice coil. Thus, there have been limits to improving sound quality and reducing power consumption of audio devices.    Patent Document 1: Japanese Unexamined Patent Publication No. 2006-174432    Patent Document 2: Japanese Unexamined Patent Publication No. 2006-33787